Magnetoelastic transducer



Jan. 2l, 1958 w. T. HARRIS 2,320,912

MAGNEToELAs-.Ic TRMISDUCER I Filed oct. 1. 1953 7j* A72 INVENTOR.

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ornsolid` niedia. Forthe higher lsonic and lsuperoiic. fanggfsh Sfllctre Het pr'tioilar i/afa; *in my coperidirig application,` Serial No. 287,077, led May/2, 17.9542, noyv'Patent 2,713,127, Iliav'eshownfliow lily "concept of variable-position transducers' may 4open the way for rnore efficient response at the lower frequencies.

liehzoelectruichresonatorsl are relatively elcient as cony ,.ofvelectrical-lenergy into acoustic energy, hut `are i ell suitewddtoI Asorrieapplicatiogis lljecausefof highfco's't, fragility, or `,sensitivity to elevated n' temperatures bariuin titanaterrochelle salt, and ammonium dihydrogenphosphate transducers rare illustrative of thoscsufferingbfrom these objections. Likewise, vmagne to `strictivetransducers ,sur ersrious handicaps, 4such as high cost,l consumption of strategic materials (i. e. nickel `or cobalt) sornewhat limited eciency conversioneciencies greater than about twenty/percent are rioteasily achieved. v .l l

, It is, accordingly, Lan object tof `this invention t .p`r'o lvide improved transducers of the character indicated.,

Ariotherrbject is to produce higher efciency electromagnetic transducers without increasing the consumption of strategic fnaterials. I s I l Y A It is a specific object to provide electromagnetic transducersachieving improvedeiciencies through reliance on 'elastic rather than magnetostrictive properties of -the core... Y, Y

It is a general object to provide a new 'organzatlionj/of partsfor the construction of electromagnetic transducers, but possessing v marked advantages over-purely magnetostrictive transducers, as well as over other known-and functionally 0comparable transducers; inthis connection, it is alsovan object to provide a rugged structurecapable ofradiatingat relatively elevated power levels. Other objects and various further features nof novelty 'and'i'rvention will be -pointed out or will occur to fthose skilled in 'theart from a` reading of the Vfollowing speciication, in conjunction with"thelTaccorrlpanying drawings. Ink sa-i'd drawings; which show?, ffor illustrative purposes only preferred forms 'of Vthefinvje'ntion I y y K V"Fig i is a Cross-*sectional view lofja transducerincorfaratgk features'of the" invention--ard revealing the plan view' of one 'of -the :laminations r'of -the -core thereof; l lFigrZ is avvie'w similar to Fig; l', but illustrating a modi'- 'ed core construction; v Fig; 3 is `l longitudinal-sectional view of a further modied core construction; Y, Figs. "4 `and f5 are fragmentaryviews:igenerallysinrilar ,Y o 'Pigsl n1 "arid 2,font illustrating'tmodied Acoreconsortia- -nding 'theinseivs "to Y'excitar-ion :fby I#an ``=ar1fay-fof windings;

2,829,912 Patented Jan. 21, 1958 a eo'fnnon centerleg, Thus,the'magieticcremay compriseftwo poterle-gs 12V-'13 and a center l115g;'having fportions 1li-lion -ppposi'te 'sides lovftlffie ."In order lto;lpr-on'iote the efficient distribution and sharingof flux, the ,Ovfll siOS-efili Pf 1 'the center fles flil'may he substantially fthe 'sum of the. cros's-sectiionsio'fr-vihe Yruiter llgs; '12,-,13am.1,.i the,` ffvrm; Shown, the. Width, f.4 the Genfsrlsg 14+f15 .iS Substantially. Wis@ the itdiidul fthe puter legs 121-13.- The Winding-19h PrefP ablylinked tthewrefmeans OnSbQth'4-SideS-Of the-'sap rifas, lin 1th@ symmetrical disposition Grown., 4, V4A. Vcom-- valets.:tra11du ,r maya-,comprise a" Stash' f lm'iiuatifms Yas described androf overall height eggtendinggqutfogtheiplane offthe dravvli rig,depending `rigori-the desiredfperformanee. A11 Hlanlrirlfitiorts may be aligned and held; "in .Compressed and-permanently xed relation-by plasticabondingand `by means of -tiejboltsf, as; at16. .L or efieient 'radiatiomin 'uid 'media,'--the #gap 11 `should aoe ,pniied--or should be *iled with-an acoustic absorber vsucl'ifas cor-k or fsponge zrobbery-'but `-forj'ruggednessV l-prefer that--al-lother voids be filledI 'oral-potted with non-magnetiqmaterial,aswith a-firulber-obpla'stic filler, designated 17, A'making 'sure to rrelieve thefclafnping eiectoff'the -plasicfas Thy-means of ya-corlc onthefhk'eliner-U'; p

In 'insg-:since I ldo VVnot y"rely'primaril-yroncmagnetcstrictiv'eperforinancm the Acore l'need V'not-tbe `*of highly rened material designed fto 'optimize .magnetostrictive response, but Trnay, Eon itheother-ih rrd, xbe merely of ordinary.siliconlfsteh as; used in t `nsiiorrner:'-flarninations. j'WhiIefthi's vrri'aterial `irlaly ossejss ESor'rje.: magnetostrictive propeftie's; theqpred'ominant:i*elongationiorfccnL "'tractionwproduc'ediuponfeitcitig nthe cere lis iatt'rib-table to the magnetic'fcontractiontofv'theiples `-1f4;-'15acr ss the gap '1'1. ,'tlnvthevcse'illstrated,fwhichilvshall-ternthe na'grietoelasticfcasq and fusing `silicon-"steel Viaminatioris with a `0.01f-cm'. 'i'gap #11,@anvincrementalelerrgth vchange fdl/*lfeigpressed'kin ter'ms ofjoverall 'length ofnth'e Fexcited flegffm-a-'y 'be oie-the'dder 'of ``=600 l`05fat8t 12000 lV magnetostrictive material (e. g. nickel) is employed, the proportional length change may be of the order of -40 10 at saturation (B=5000, H=500, approximately).

Noting that the net magnetic permeability for the silicon-steel case, including the eiect of the liux gap, may be greater than the usual permeability of commercial nickel, and that the saturation tiux density is three times greater, it is apparent that, on a comparable-size basis, magnetoelastic transducers can handle approximately ten times the power conversion of the nickel purely-magnetostrictive type. Further, since high-quality transformer iron has a ten-fold higher resistivity, eddy-current energy dissipation, the chief cause of low eiliciency in magnetostrictive transducers, is far less important. Also, transformer iron may be cut burr-free in properly made press tools, whereas nickel is tough and ductile, and the resulting edge fins tend to produce short-circuits between laminations. Indeed, by resorting to my magnetoelastic transducer construction, the usual 20 to 25 percent etliciency for magnetostrictive transducers can be increased to substantially 75 percent, while effecting important economies.

With further reference to the specic case discussed in connection with Fig. 1, it is advantageous to make the iiux gap relatively narrow, generally less than 0.005 inch. This can be accomplished economically by so designing the power-press tool that the laminations are cut in three operations or stages, which follow each other in automatic sequence: (1) cut out the spaces where the winding will be placed; (2) shear the center leg apart without making a gap; and (3) shave one of the sheared edges to produce the desired ux gap.

In further comparing my magnetoelastic transducer with magnetostrictive transducers of corresponding size, it is important to note characteristic differences which make for broad-band response. First, the resonant mode of operation occurs at a frequency substantially below the fundamental; the motion is characterized by an oscillatory inward and outward bowing of the outer legs 12--13, and with D. C. magnetic polarization the structure can be mechanically biased to produce essentially only outward bowing, thus avoiding frequency-doubling effects, and producing a limited broad-band response in the range below the fundamental. Second, the magnetoelastic transducer basically comprises two coupled lengthwise oscillators, not just one; on a lumped-constant basis, one lengthwise oscillator may be visualized as comprising two end masses connected by longitudinally compliant outer legs 12-13, while the other lengthwise oscillator (coupled to the first) is two-fold degenerate and is the longitudinal mode of the two halves 14-15 of the center leg. The two coupled resonant systems result in a net combined characteristic having one resonance lower in frequency than the lower of the two (when decoupled and viewed independently) and another higher resonance higher in frequency than the higher decoupled resonant frequency, and a broad high-response region between the two resulting resonances. Such an eicient broad-band response is a total impossibility with any known magnetostrictive element.

Because of the development of winding in the construction of Fig. 1 may be difficult and tedious, I show in Fig. 2 an alternative construction lending itself to relatively simple manufacture. .In the construction of Fig. 2, the core may in all respects resemble that of Fig. l, except that it is made in two like halves. The two halves may be of generally E-shape with the legs facing each other and with the center leg slightly shorter than the outer legs, so that when the outer legs are anchored together, a gap of desired proportions is formed in the center leg. However, in the form shown, the two halves of the basiccore laminations are of generally F-shape, comprising (in the case of the left half) what may be termed an elongated body 20, with a transversely extending leg 21 integral with one end of the body 20, and with a second transversely extending leg 22 integral with the center of the body, but of effective length just less than one-half the etective length of leg 21; corresponding parts of the right-hand half of the laminations have been shown with the same reference numerals primed.

The construction of Fig. 2 lends itself to the separate winding of coil 23 and to the subassembly of the two halves of the core as separate consolidated stacks. Control of the size of the flux gap presents no substantial problem, since this is a matter of proper tool dimensioning; however, great care must be taken, when consolidating the unit, that plastic is not allowed to run into the flux gap. An acceptable method of assembly is first to consolidate with a plastic bonding agent two F stacks in suitable jigs, and then to clean off the consolidated stacks by filing or milling any deposits from surfaces that abut, as well as from the flux-gap surfaces. The consolidated coil assembly 23 may then be placed on one F, and the other F fitted in place. The device may tinally be fastened together by a compression band or by straps and rivets, as, for example, end straps extending between tie bolts 24 and between tie bolts 25, respectively.

For consolidating the laminations I prefer to spray the laminations individually, taking care that parts near the flux gap are shielded, and employing a thin solution of a high-temperature-curing epoxy resin. The sprayed pieces dry quickly and can be separately handled and stored. When later assembled in presses and baked, the sprayed pieces bond into a solid structure.

In Fig. 3, I show that my invention is not inherently limited to laminated or to multiple-leg core constructions, as suggested in Figs. 1 and 2; in fact, in Fig. 3 I show what I term a basic magnetoelastic core construction 'for insertion in any desired ux path. The basic construction of Fig. 3 involves a plurality of like core elements 30, which may be cylindrical, tiat, square, or otherwise formed, but having a section with the appearance shown. The core element 30 may be of magneto strictive material, and I have illustrated by the sectioning my preference for a high coercive-strength material, which may be one of the ferricoxide ceramics known as ferrites. The elements 30-31 are preferably arranged in end-to-end relation with gaps (as at 32) of desired proportions between adjacent ends, and non-magnetic means may be employed to space the elements Sil-31. In the form shown, relatively rigid spacers such as ceramic blades or tubes 33, as the case may be, are set in direct longitudinally spacing abutment with outwardly projecting anges 34 at the central portions of the elements 30-31; and shoulders 34 at flanges 34 may provide proper transverse or radial locating support for spacers 33. The spacers may be bonded, as by thermosetting plastic means, to the supporting shoulders for the magnetostrictive elements .3D-31, and a completed end-toend asembly of any desired number of elements may readily be fabricated. By means of the dot-dash line 35, I suggest a continuous flux loop utilizing the described multiple-element assembly as a part thereof; the line 3S will also be understood to suggest that the rest of this flux loop or magnetic circuit may include further successions of elements 30-31. By exciting one of the elements 30-31, as by coil means of limited length, and by picking olf a signal at a remote element, as by a further coil of limited length coupled to a remote element 36-31, I provide adaptation of my multi-section core to delaylay functions; but, in the form shown, the phantom-outline coil 37' schematically suggests that, for other purposes, a plurality of elements 30-31 may be energized by a single coil.

In Figs. 4 and 5, I show application of the principles of my invention to a multiple-winding'transducer in which the windings may be connected by known methods in order to develop array properties in the transducer face.

a dut-"6r" the plan ofthe drawng'; breakingfo'rie'f theseco'nnectionsand placing the signal p 'liaraiteriz b plurality of 5Vv generatori'r" receiver, asthe case'v may be, in'serie's"w`ith longtidi'nally paced, trarifsvse`ly"exteniirign pairs" of all windings. lf desired, the voids between' ferrornag2 43444; exterridi'ri'g" liirtif'tl'i sdsv"40i`-41` and ntic' elements and the inner and 'outer walls SS--Siniay alternate legs 45, leaving the other legs` 46 continuous tially extending flux path willresult in simultaneoiiselon- A ""f th c'FL Separate electrical gaton `or coritraction', asthe' caseV may be, of all ferromagnetic eneirrentsy 56'-\57;59, as' accentuated'yth magne'relasttcphenomenon that is characteristic cfthe invention. Because ci th cylindricarnature'v or thejcoiii M etionwitliFig figuration,` these elongations will be accompanieduby ilthe" configuration 'off Fig!v 4',frsultrngir`i` afro'nt-tojb'ack fliic'tutingfradial pressures or forces applied tothespcer C??? Y, S9111. f the transducer *Suche Injicis salga/nich' inY tummy serve a@Y pistons driving ld, of

spaecdfp'rts f the respective walls 55;.56. lif'pry dominantly radiallyy inward propagation is desir'ed,l Athe a'pplic'ath ofl a pressure-release` blanket, a'siof air-foam rzbcr; te iiidurer waii'ss` will Servera a-'cc'er i l faariryfiinvaia'rniation; inrike manner, maiali hfe ySlf' a um?V wardvresponse may be promoted by nle'aiis of a'fsiriiilr i dlfgsisos 25 blinl l fi the inner surface' of the inner wallV 56;`

` I '7y 'lustratethat my basic cons'truciti n nent polarizing, even when the laminat las inV Figs. l, 2, 4, and 5 are of relatively r'ic'e material, as for example, the'ordiriary sst elfused for transformer cores.' 4ln Fig. 7,fperp "polarization is achieved byjpartia'lly" filling a enlarged flux gap 70in the center legf 71` vv h atrahsversely extending slabV 73"o` f higlilcoe'rciv forcefrr'i'ate'rial,`l such as a inagneti'cxide cerar'i'iic,y Of ours, in placing the slab 73 in the' gap 70,'gre'atl ca nfiust'hbef taken to leave a sullcient Aresidual gap for fnagiie't'ela'sticv contraction and to assure such spa' ng as will avoid harm to the ceramic 73. For protection of rai'nic 73, l have illustrated the employment of r U vly thin pressure-release layers 74, which be of cardboard or cork, cemented to opposite sides ofthe ceramic 73 before insertion as a preassembled sandwich infile ga'pvo. m Y e `Although the gap-insert configuration of Fig. 7

with important economies in lamination materiaLkthe Structures of Figs. l, 2, 4 and 5 will be understood to lend themselves to permanent polarization upon'resort to the relatively expensive expedient of employing ferroe magnetic materials with sufficiently high coercive force, andin that event no special permanent polarizing means need beyemployed; moreover, as long as the flux gaps are made sufficiently narrow, satisfactory polarization may be achieved with laminations of only moderatcoercive-force materials, thus permitting the use of high= permeability materials for the lamination. l

As noted above, any magnetostrictive `eiect's con# current with my magnetoelastic effects may'reinforceor einer icsrbprrthe elements s145845; i'hve' eo @Umm/.negative the maglemiastic etts" ,d epd sh wa @naar Snif spacer naar 6s which' may' be @the S1?? 0f thamgnemsfflcwe Comi ,of tilt??? iiyicficgiiidrfiiy ccxnfsive with the elements @L t Wm? matenal (sich aslmckei? h aYmSaBf'ga-@Yf' s sn an' which serve t space Cem-fai patrona what the magnemsflctw? @d flagPetOlelsnc. effec? fmfra'idiauy varia fsp'ecttth twradatiflg W41 ald each 0th,, Pfoflvcmg deslfeble #esula at.. F11? viali-s" S "S. I prefer that these` spacer' blocls be 65 expense 0f.st.rateg1c matenals -iiqwever foftrilnsilmer. iaad soir animalista@ ta the ferromagnetic are.' Steel for which the manfosffme Constante Per if faena; aaai' isfairfd'fiaiiay ceramic materia far' frias (and therefore amagommc to. magnefeelaeuc @their Also to facilitate assembly, but yet/nlotbtouim'p'ir the veiau performance, may Suu be Prferrd. overttilat 1. .f :ce ffii oi' a nickel core because of the higher permeability,

higherl saturation-ux density, and higher electrical re'- sistivity` obtainable. l j VIt will be'yseen'that I have'describedb i l nstriictionsl characterized by'wrelativ'ely' high e''icf, pt la'ly'fresponsetoupress 75 r in solids". My'use of theniagnetoe'lastic principle pro in certain caseslprovide permanent polarization, together' vides increased power-conversion potentialities, which may be in the order of at least ten times those achieved with purely magnetostrictive configurations made with the best magnetostrictive materials. Furthermore, I achieve my improved performance with a minimum use of scarce materials.

While I have described the invention in detail for the preferred forms illustrated, it will be understood that modifications may be made within the scope of the invention as defined in the claims which follow.

I claim:

l. In a transducer of the character indicated, a threelegged ferromagnetic core with an open gap in the center leg, said gap being substantially shorter than the spacing of said center leg from either of the other legs, and an electrical winding coupled to said center leg.

2. A transducer according to claim 1, in which said gap is intermediate the ends of said center leg, and in which said winding is coupled to said center leg on both sides of said gap.

3. A transducer according to claim 1, in which the minimum cross-sectional area of said center leg is substantially twice the minimum cross-sectional area of the outer legs of said core.

4. A transducer according to claim l, in which the effective minimum cross-sectional area of said center leg is substantially equal to the sum of the minimum effective cross-sectional areas of the outer legs of said core.

5. In a transducer of the character indicated, a core defined between opposed spaced sides and having a plurality of internal slots extending between said sides but short of said sides to define a plurality of spaced legs extending between said sides, alternate of said legs having gaps therein, and winding means supported in slots adjacent each gap and linked to the legs having gaps.

6. A transducer according to claim 5, in which the opposed sides of said core are substantially flat and parallel.

7. A transducer according to claim 5, in which said opposed sides are substantially arcuate about the same center so that acoustic response may extend radially of said center.

8. In a transducer of the character indicated, a closed magnetic circuit including a core of ferromagnetic material and having a gap therein, magnetizing means in said gap and of an effective thickness less than the width of said gap, and electric winding means linked to said ferromagnetic material.

9. A transducer according to claim 8, in which said magnetizing means is a permanent magnet.

10. A transducer according to claim 8, lin which said magnetizing means is a high-coercive-force material.

11. A transducer according to claim 10, in which said material is a magnetic-oxide ceramic.

12. In a transducer of the character indicated, a closed magnetic loop of elements of ferromagnetic material spaced in end-to-end adjacency to define a plurality of spaced gaps, and electric winding means linked to said ferromagnetic material on opposite sides of said gaps.

13. In a transducer of the character indicated, a loop of non-magnetic material defining a radiating wall, a plurality of closely spaced ferromagnetic elements substantially coextensive with said loop on one radial side thereof, relatively rigid supporting means supporting central portions of said elements in radially spaced relation with said loop, and electric winding means coupled to said ferromagnetic elements on both sides of the gaps defined by the spacing between elements.

14. In a transducer of the character indicated, a cylindrical radiating wall, a plurality of ferromagnetic elements in closely spaced relation with each other and longitudinally substantially coextensive with said wall, rigid supporting means spacing the central portions of said elements radially inwardly of said wall, and electric winding means coupled to said ferromagnetic elements on opposite sides of the gaps defined by the spaces therebetween.

15. In a transducer of the character indicated, a cylindrical radiating wall, a plurality of ferromagnetic elements in closely spaced relation with each other and longitudinally substantially coextensive with said wall, rigid supporting means spacing the central portions of said elements radially outwardly of said wall, and electric winding means coupled to said ferromagnetic elements on opposite sides of the gaps defined by the spaces therebetween.

16. As an article of manufacture, a magnetoelastic core subassembly comprising a plurality of elongated elements of ferromagnetic material, and non-magnetic means relatively rigidly spacing central portions of said elements and aligning the ends of said elements to define gaps therebetween in an end-to-end alignment of said elements.

17. As an article of manufacture, a magnetoelastic core comprising a plurality of elongated ferromagnetic core elements, each of said coreelements having an enlarged central shoulder extending transversely of the longitudinal axis thereof, and rigid non-magnetic spacers supporting said elements in end-to-end relation with a gap between adjacent ends and in direct longitudinal thrust-sustaining abutment with said shoulders.

18. As an article of manufacture, a magnetoelastic transducer lamination of ferromagnetic material comprising three legs defining two slots adjacent a center leg, said center leg having a flux gap therein and being of an effective width substantially twice that of each of the outer legs, said gap being substantially less than the width of either of said slots, whereby said center leg including said gap may form an important part of a magnetic circuit when an electric winding is coupled thereto.

19. As an article of manufacture, a magnetoelastic transducer lamination of generally F shape, comprising an elongated body, a first leg extending integrally from one end of said body and transversely therefrom, and a second leg extending substantially parallel to said first leg from the center of said body and to an extent slightly less than one-half the length of said first leg, whereby two such laminations may be fitted together to define a three-legged core with a gap in the center leg.

20. In a transducer of the character indicated, two mechanically coupled mechanical oscillators of magneticflux-conducting material, said first oscillator comprising two end masses with an elongated longitudinally compliant leg connecting said end masses, said second oscillator comprising an elongated longitudinally compliant leg directly connected to one of said masses to the exclusion of the other, the compliant axes of said legs being similarly oriented, said second-mentioned leg being coupled to the other of said masses by means including a magnetic-fiux gap, and means establishing varying magnetic flux in said second-mentioned leg.

21. In a transducer of the character indicated, a magnetic circuit comprising two mechanically coupled mechanical oscillators of magnetizable material, said first oscillator comprising two end masses with an elongated longitudinally compliant leg connecting said end masses, said second oscillator comprising an elongated longitudinally compliant leg connected to one of said masses to the exclusion of the other and completing the magnetic circuit to said other end mass through a magnetic-flux gap, and an electrical coupling to said circuit for establishing magnetic-flux variations in said second-mentioned leg and across said gap.

22. In a transducer of the character indicated, a magnetic circuit comprising two mechanically coupled mechanical oscillators of magnetizable material, said first oscillator comprising two end masses with an elongated longitudinally compliant leg connecting said end masses, said second oscillator comprising two elongated longitudinally compliant legs connected respectively to said end masses and extending toward each other to define a generally central magnetic-flux gap, and an electrical coupling to said circuit for establishing magnetic-iiux variations through the two legs and gap of said second oscillator.

23. In a transducer of the character indicated, a magnetic core comprising a consolidated stack of like laminations of ferro-magnetic material, each lamination including three legs joined to each other at opposite ends and defining openings between legs, the center leg having a magnetic-ux gap therein of spacing substantially less than the Width of said center leg, the open span of said gap being sufiicient to permit free longitudinal oscillation of the poles at said gap, and a winding passing through said openings and coupled to said center leg.

24. A transducer according to claim 23, in which said gap is centrally located on said center leg.

25. A transducer lamination, comprising a single sheet of ferro-magnetic material having two spaced, like, and symmetrically located winding openings therein, said sheet having a thin elongated open flux gap communicating with both said winding openings, thereby deiining two poles in a center leg at said gap, whereby an electric winding received in said winding openings and coupled to said center leg may be effective to create fluctuating magnetic forces at said gap, and means independent of said center leg for supporting said lamination, whereby said poles have maximum freedom for elastically restrained relative longitudinal displacement upon development of such forces.

26. A magnetoelastic transducer, comprising a consolidated stack of like ferro-magnetic laminations, each lamination having two spaced winding openings therein deiining rst and second flux paths around said respective openings and in common with the part of said lamination between said openings, each lamination having a flux gap connecting said openings and defining spaced free poles on opposite sides of said gap, the width of the lamination along said gap being substantially twice the width of each said iiux path external to the part between said openings, and a Winding with turns passing through said openings and linked to said part between openings, whereby said winding may be effective to create fluctuating magnetic forces at said gap.

27. A magnetoelastic transducer, comprising a complete iiux-path circuit of ferro-magnetic material, said circuit including two like elongated inherently longitudinally vibratile members aligned in end-to-end relation, said members being closely spaced and supported freely at their adjacent ends to define a flux gap, and a winding coupled to said flux-path circuit, whereby the longitudinal oscillatory mode of said members may directly determine the span of said gap, so that upon application of oscillatory driving energy to said winding the magnetic forces at the gap may directly stress-excite said members into longitudinal-mode oscillation.

References Cited in the tile of this patent UNITED STATES PATENTS 252,256 Rogers Jan. 10, 1882 1,951,018 Herdman Mar. 13, 1934 2,152,955 Coyne Apr. 4, 1939 2,437,270 Peek Mar. 9, 1948 2,445,088 Schilling July 13, 1948 2,636,135 Peek Apr. 21, 1953 FOREIGN PATENTS 704,699 Germany Apr. 4, 1941 

